The present invention relates to a bottom closure for a container. More particularly, it relates to a bottom closure of elastic material, which is utilized as a pushing piston for viscous mass for a hollow cylindrical extrusion container, particularly for a cartridge.
Bottom closures of the above mentioned general type are known in the art. A known bottom closure has a closing disk located adjacent to the storage chamber of the container, and a hollow guiding portion extending from the closing disk toward a filling opening of the container and provided with at least one elastic lip which surrounds this hollow guiding portion and abuts against the inner wall of the container. When extrusion containers or cartridges are filled with permanently elastic, permanently plastic, hardenable and other similar paste-like materials, particularly filling masses, it is necessary to utilize the above mentioned pushing pistons which serve simultaneously as bottom closures of the containers before the utilization of the plastic masses. Such bottom closures are described, for example, in the German Pat. No. 2,034,047 and in the German Offenlegungsschrift No. 3,005,855 which corresponds to U.S. patent application Ser. No. 227,410. The above mentioned bottom closures are composed of a maximum elastic synthetic plastic material for providing the elasticity required for all functions of the bottom closures, for example of polyethylene or polypropylene. In practice, it has been recognized that such specially elastic synthetic plastic materials have a relatively high gas and water vapor permeability. However, it is frequently required to fill the extrusion containers, and particularly cartridges with masses which contain volatile components, particularly solvents. In the event of the above mentioned gas permeability, these components can diffuse through the walls of the piston, so that the required tightness of the bottom closure, particularly over long storage times for such masses, is not guaranteed, and these masses gradually experience not acceptable changes, for example premature hardening. When instead of the above mentioned highly elastic synthetic plastic materials, other synthetic plastic materials are utilized which have the required gas diffusion tightness, it has been recognized that these synthetic plastic materials simultaneously have such an increased hardness and rigidity that the elasticity required for functioning of the bottom closures and pushing pistons is not guaranteed. In other words, the ventilating function during insertion of the bottom closure, and stripping and sealing action during loading of the bottom closure as a pushing piston, are not provided.